1. Field of the Invention
The present invention relates to an integrated circuit device which is used in a display apparatus in order to process an image signal which is supplied to the integrated circuit device from a computer.
2. Description of the Background Art
In general, in order to output an image, a computer provides a display apparatus with information about the image in the form of three different signals Red, Green and Blue (hereinafter "R, G and B"). The display apparatus receives and processes these signals to display the information supplied to the integrated circuit device from the computer.
Sometimes, one display apparatus is shared by a plurality of computers. In such a case, the display apparatus includes a plurality of connectors so that the display apparatus can select a computer which is connected to any one of the connectors.
There are various connection methods for connecting the display apparatus and the computers. This is because there is no standard with respect to a connection method for connecting display apparatuses and computers. Rather, there are various standards to deal with various kinds of image signals of a variety of characteristics such as different frequencies outputted by the computers. To make it possible to connect a display apparatus with computers which are designed in conformity with different standards for connection, in most cases, the display apparatus includes a plurality of connectors of different configurations.
Since the display apparatus is to process signals supplied through the connectors of the computers which output information, the connectors and signal processing parts of the display apparatus must be connected with each other within the display apparatus. To this end, a change-over switch must be disposed for selecting connections between the respective connectors and the signal processing parts (i.e., preamplifiers).
To display an image, the display apparatus needs an image signal which consists of three color signals R, G and B as well as a synchronization signal. There are two methods of outputting a synchronization signal from a computer to the display apparatus, one in which a synchronization signal is transferred on a signal line which is disposed independently of a signal line for transmitting an image signal, and the other in which a synchronization signal is superimposed on one of the signals R, G and B. In the latter method, in general, a synchronization signal is superimposed on the signal G. The signal G must be supplied to a synchronization separation circuit as well which is the first signal processing part of a synchronization circuit in this case.
FIG. 17 shows a circuit structure of a portion of the display apparatus where a signal is processed. In FIG. 17, indicated at numerical reference 201 is a connector which is connected to a computer or the like to receive an image signal which consists of three signals R.sub.1, G.sub.1 and B.sub.1, indicated at numerical reference 202 is a connector which is connected to a computer or the like to receive an image signal which consists of three signals R.sub.2, G.sub.2 and B.sub.2, indicated at reference characters c11 to c13 are capacitors for cutting direct current components of the signals R.sub.1, B.sub.1 and G.sub.1, and indicated at reference characters c14 to c16 are capacitors for cutting direct current components of the signals R.sub.2, B.sub.2 and G.sub.2. Indicated at numerical reference 204 is a relay for receiving the signals R.sub.1 and R.sub.2 and outputting one of the two signals in accordance with a signal which is received at its control terminal, indicated at numerical reference 205 is a relay for receiving the signals B.sub.1 and B.sub.2 and outputting one of the two signals in accordance with a signal which is received at its control terminal, and indicated at numerical reference 206 is a relay for receiving the signals G.sub.1 and G.sub.2 and outputting one of the two signals in accordance with a signal which is received at its control terminal. A symbol SW4 denotes a switch for switching the control signals which are supplied to the relays 204 to 206. A preamplification part (hereinafter "preamp part") formed in one chip to amplify a received image signal to an intermediate level is indicated at 203. By means of a switching operation of the switch SW4, the levels of the control signals which are supplied to the relays 204 to 206 are switched simultaneously between a high level (a power source potential supplied from a direct current power source E5) and a low level (a ground potential). Hence, by switching the relays 204 to 206 at the same time, either signal set of the color signals R.sub.1, B.sub.1, G.sub.1 or R.sub.2, B.sub.2, G.sub.2 is supplied to the preamp part 203. The relays 204 to 206 are each formed by parts which are not shared by the preamp portion 203.
In FIG. 17, power amplification parts (hereinafter each "poweramp part") are indicated at 208, 209 and 210 which amplify and output the signals R, B and G, respectively, which were already amplified by the preamp part 203. Indicated at numerical reference 211 is a deflection circuit for creating a current which horizontally and vertically deflects an electron beam which is used to display an image on a screen from the synchronization signal which is included in the signal G. Indicated at numerical reference 212 is a synchronization separation circuit for separating the synchronization signal from the signal G which is supplied to the deflection circuit 211. Indicated at numerical reference 213 is a horizontal deflection circuit for generating a sawtooth current used for horizontal synchronization from the synchronization signal which is outputted from the synchronization separation circuit 212. Indicated at numerical reference 214 is a vertical deflection circuit for generating a sawtooth current used for vertical synchronization from the synchronization signal which is outputted from the synchronization separation circuit 212. Indicated at numerical reference 215 is a CRT which is connected to the poweramp parts 208 to 210 and the deflection circuit 211 to display the image signal. The CRT 215 includes electron guns 215a, 215b and 215c which emit electron beams under the control of outputs of the poweramp parts 208, 209 and 210, respectively. The CRT 215 also includes a horizontal deflection coil for horizontally deflecting the electron beams under the control of an output of a horizontal deflection circuit 11 and a vertical deflection coil for vertically deflecting the electron beams under the control of an output of a vertical deflection circuit 12.
In general, the preamp part is formed as an IC. For this reason, in some cases, the preamp part is formed to include independent parts for respectively processing the signals R, G and B; that is, the preamp part is formed by three ICs. In the display apparatus of FIG. 17, the preamp part 203 is formed by one IC which is capable of processing the signals R, G and B at the same time. The preamp part adjusts a contrast between the signals R, G and B and amplifies the voltages of the signals R, G and B so that the signals R, G and B have amplitudes sufficiently large as inputs to the poweramp parts. The deflection circuit 211 is formed independently of the preamp part 203.
In the conventional display apparatus having such a structure as above, either the signals R.sub.1, G.sub.1, and B.sub.1 or the signals R.sub.2, G.sub.2 and B.sub.2 from the two connectors 201 and 202 are selected by the relays 204 to 206 and provided to the preamp part 203 while selecting and supplying either one of the signal G.sub.1 and the signal G.sub.2 to the deflection circuit 211.
For instance, mechanical relays are used as the relays 204 to 206. However, the use of mechanical relays has a problem that an inductance, a resistance component and a floating capacitance component against ground from the input to the output of the relay, are large beyond allowable levels with respect to a signal frequency (of 100 to 200 MHz, for example). This eventually deteriorates the frequency characteristics of the signals which are transmitted through the relays 204 to 206. Although the inductances and the like of the relays must be reduced by using an expensive material for contacts or insulators of the relays and suppressing deterioration of the frequency characteristics, since manufacturing of such relays is difficult, use of such relays in the display apparatus increases the overall manufacturing cost.
To a further disadvantage, use of such relays in the display apparatus requires longer signal lines to connect the relays 204 to 206 with the preamp part 203. This causes interference between the signal lines and other circuits. As a result, the signal R, G or B adversely effects the other circuits as noise.
In addition, since the signal G includes the synchronization signal, it is necessary to connect an output of the relay 206 with the synchronization separation circuit 212. To this end, the relay 206 needs be designed to have a different impedance matching from those of the relays 204 and 205.
When these circuits are all integrated as electronic circuits of an IC, the resulting IC includes both an image signal processing part (i.e., the preamp part 203 and the poweramp parts 208 to 210) and the deflection circuit 211. This excessively intensifies interference between the image signal and increases a deflection signal and a consumption power.